Progress of magneto-optical ceramics

The magneto-optical effect (Faraday effect) was discovered in the middle of the 19^th century. In the latter half of the 20^th century, the practical use of isolators using single crystals (Faraday rotators) using the melt growth method began. One century after Faraday's discovery of the magneto-optic effect, R.L. Coble proved translucency of polycrystalline ceramics. Ceramics may have many scattering sources due to their polycrystalline microstructure, and even from the viewpoint of scattering theory, it was considered impossible to apply them to the generation of coherent light (laser). However, 40 years later, A. Ikesue demonstrated laser ceramics for the first time with performance comparable to that of optical single crystal counterparts. The possibility of laser application of polycrystalline ceramics also makes it possible to apply it to Faraday rotators (optical isolators) that utilize coherence light. A magneto-optical single crystal composed of a single crystal orientation was considered to be superior in that it provided excellent optical performance and an accurate Faraday rotation angle. However, polycrystalline ceramics composed of random crystal orientations can not only provide accurate Faraday rotation angle but can also have a higher extinction ratio than single crystal isolators. A ceramic medium with extremely low scattering and extremely low insertion loss, which cannot be achieved with a single crystal material, has been developed. In addition, new materials, which have Verdet constants several times higher than those of main commercial crystal for isolator, have made it possible to reduce the size of isolator devices. However, these materials cannot be synthesized by the conventional melt-growth method. In the 21^st century, polycrystalline ceramics are paradigms for Faraday rotating elements, and are about to enter a period of change from single crystals to polycrystalline ceramics.